There are two large groups of solid-state radiation detectors, which dominate the area of ionizing radiation measurements, namely, scintillation detectors and semiconductor diodes, see G. F. Knoll, Radiation detection and measurement, John Wiley & Sons, 2000.
The scintillators register the event of interaction with a penetrating high energy radiation through the generation of light which is subsequently detected by a photo-detector, typically a photo-multiplier, which converts light into an electrical signal.
Another group of solid-state detectors is based on semiconductors and employs reverse biased diodes, typically, p-n junctions, wherein the absorbed radiation creates in the depleted region of the junction a current of electrons and holes thereby producing an electrical response signal. The sensitivity of such detectors depends on forming a large active volume for interaction with the radiation, which is determined by the depleted region of the p-n junction. To increase said active volume, the doping level in the depleted region of the p-n junction sides must be minimized, since the depletion length is proportional to 1/(N)1/2, where N is the majority carrier density (electrons in n-type or holes in p-type).
Both groups of detectors have their drawbacks, resulting in a lower than desired signal response and resolution. The diodes typically suffer from inadequate electron-hole collection, i.e. not every electron-hole pair created by the radiation results in a current flow in the measurement circuit. In the case of scintillators, the efficiency of converting the high-energy radiation into light typically does not exceed 12%. In addition, the recombination time that is involved in light emission is several hundreds of nanoseconds (e.g., 230 ns for NaI), which is undesirably long for fast timing or high counting rate applications. Finally, all commercially available scintillators have a high energy gap, and therefore a relatively high energy (of 25 eV for NaI) is required per each electron-hole pair created by the primary ionizing radiation, which reduces the detector resolution.
The most common semiconductor materials used for the radiation detectors are Si and Ge, where the intrinsic carrier concentration can be reduced to a very low level, while the excellent material properties provide for good electric field uniformity. However, the relatively low atomic number Z of these materials, especially for Si (where Z=14), adversely affects their application to radiation detection, since the probability of interaction is proportional to Z4.5. In order to obtain an acceptable p-n junction depletion length of approximately 1 to 1.5 cm, an additional procedure of Li doping is commonly applied to neutralize acceptors in the depletion region. The Li-doped detectors, however, need low temperatures, both during the operation and in storage. In addition, both Si and Ge radiation detectors require relatively high voltages, typically of order kilovolts, to maximize the collection of electrons and holes and increase their drift velocity. This results in an additional unwelcome noise in the current response, as well as leads to problems of surface conductance and voltage breakdown. Even at these high voltages, the response time is larger than 100 ns, because of the saturation of the electron and hole drift velocity at high fields. Finally, the dependence of the shape of the output pulse rise on the position at which the electron-hole pairs are created, significantly complicate the measurements.
Both above discussed groups of detectors are so different in their physical mechanisms of signal registration that they never overlap with respect to the materials used.